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Choice: No Vapor Barrier In Bath, Or Mold Sandwich?

 
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Old 05-31-2011, 09:52 AM   #1
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Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Gentlemen,

I've got a bath/shower all ripped out and ready to rebuild. Stop and think a second - behind the wall there's a shower that overlaps this bath by two studs. The shower was done in Hardi board with Kerdi overtop, so it has no vapour barrier - but the Kerdi sure has the same effect.

The debate is on this side - on the one hand, not putting up vapour barrier behind the Hardi is just wrong. On the other hand, with the Hardi and Kerdi on the other side, that would be creating the dreaded double-vapour-barrier mould sandwich in those two cavities.

Hmmm..which is the lesser of two evils?

Last edited by griz; 05-31-2011 at 05:22 PM.
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Old 05-31-2011, 01:12 PM   #2
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


You can use NobleSeal TS instead of Kerdi on the new bathroom. It is a vapor barrier. Kerdi is not.

Problem solved.

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Old 05-31-2011, 01:26 PM   #3
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Appreciate the input. But the question isn't how to apply a vapour barrier so much as whether or not to considering there's already one on half the wall from the shower on the other side.
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Old 05-31-2011, 01:39 PM   #4
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Quote:
Originally Posted by bob_cntrctr View Post
Appreciate the input. But the question isn't how to apply a vapour barrier so much as whether or not to considering there's already one on half the wall from the shower on the other side.
I didn't reply on how to apply a vapor barrier, I suggested using NobleSeal TS in the new bathroom. the NobleSeal will take care of any moisture getting into the cavity from the new side. The old side with Kerdi will act as it has been since installation.

My point, by using NobleSeal, you are not contributing any more vapor into the wall bay.
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Old 05-31-2011, 01:59 PM   #5
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


OK - hate to be contrary - but, maybe I'm missing your point.

Two bays on the other side of the wall have Hardi with Kerdi. We never put vapour barrier behind Kerdi because we consider that will create a vapour-barrier sandwich. That is, that wall is considered to have a vapour barrier on it.

From the Schluter website:

http://www.schluter.com/5288.aspx

"In addition, KERDI protects wall cavities from vapor penetration."

and

http://www.schluter.com/8_1_kerdi_6550.aspx

Q. Can I install Schluter-KERDI over drywall in my tiled shower?

A. Yes. Once you install Schluter-KERDI over the drywall panels, they will be completely protected from the water and vapor in your tiled shower.

But we DO usually put poly vapour barrier behind Hardi when it's not going to get Kerdi over it. (Although I;ve even seen debates about the absolute necessity of that.)

But, whether I apply NobleSeal or poly to the new wall, in either case I'm adding a vapour barrier to this other side and creating a vapour-barrier sandwich - that is, vapour barrier on both sides of the wall, which is a no-no.

No??

Last edited by bob_cntrctr; 05-31-2011 at 02:05 PM.
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Old 05-31-2011, 01:59 PM   #6
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Can you coat the back side of the existing showers Hardi board and studs with a couple of coats of RedGuard or Laticrete Hydro-Ban? That way, if any moisture did collect in the stud cavity, the wood would be protected I'd think. And, AFIK, mold won't grow on the Red Guard and/or Hydro-Ban. If there is an attic above the bathrooms, could you run a 1/2" rigid PVC tube through the top plate of the stud space long enough to clear any insulation in the attic, to allow any moisture to escape the cavity?
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Old 05-31-2011, 03:03 PM   #7
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Quote:
Originally Posted by bob_cntrctr View Post
But, whether I apply NobleSeal or poly to the new wall, in either case I'm adding a vapour barrier to this other side and creating a vapour-barrier sandwich - that is, vapour barrier on both sides of the wall, which is a no-no.

No??
Kerdi has a listed PERM rating of .75. That is at ASTM E96 B. If tested at a more critical ASTM E96 E, Kerdi is actually 1.27.

NobleSeal at ASTM E96 E is .05.

So if you want to get technical, Kerdi is not a vapor barrier based on ASTM E96 E but NobleSeal is.

So as I am saying, by using NobleSeal you are not adding any vapor in that cavity. If you believe the Kerdi is doing its job, there should be no vapor in that cavity for a "moisture sandwich" to occur. I mean do you understand what my point is? Where would the vapor come from that makes you worry about the sandwich?
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Old 05-31-2011, 03:19 PM   #8
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


If you have back to back (two bays worth) of shower wall isn't it an inside wall cavity not an exterior wall? Won't the wall be a little warmer being an interior wall?

If this isn't a steam shower, I don't believe there will be enough steam to penetrate into the wall cavity. I've never seen and no membrane manufacturer has ever shown me a picture of the dreaded moisture sandwich. Maybe in a health club where the steam is constant and for hours but in a home, I have trouble believing in this. Just my point of view.
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Old 05-31-2011, 04:17 PM   #9
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


OK, then screw it. No vapour barrier at all behind the new Hardi. I'll just seal the joints and screw heads with kerdi band to ensure no liquid water can get back there, confident that no moisture vapour is getting back there anyway.
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Old 05-31-2011, 05:16 PM   #10
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Take it easy Bob. Vapor and moisture are two different things. I use liquid membranes all the time. I don't want moisture in the wall cavity.You wanted to know if vapor will get into the wall. Different animal. I think residential steam/vapor is overated unless it's an actual steam shower.
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Old 05-31-2011, 06:04 PM   #11
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Must......resist.....overthinking.......must...... .resist......

So, to summarize:

- When we build a shower with Kerdi over Hardi, we don't use a poly vapour barrier behind the Hardi. Some of us thought that was to avoid the vapour-barrier-sandwich, but it might not be the case.

- Where would vapour come from anyway? Especially in an interior wall.

This seems to be leading to why ever bother with a vapour barrier behind concrete board on an interior wall.
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Old 05-31-2011, 08:37 PM   #12
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Bob this is my take on the matter and you can do whatever pleases you.

Personally, and it's just my opinion, I think the plastic sheeting behind cement board was a good thing before liquid membranes came out. But nails through the board into the plastic sheeting weren't so great. Now, you can waterproof the surface of the backer and the nails so water can't get into the wall.

Vapor is a different issue. How long can a shower be used before the vapor enters the wall cavity in an open shower/tub? How long if you had a shower curtain vs. a glass door? Where should the vapor barrier be in a cold climate vs. hot humid climate with air conditioned spaces? Shouldn't an exterior wall be more important than a warm interior wall?Shouldn't residential areas be more forgiving of vapor than your health club's constant use? Shouldn't we just CYA and move on?
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Old 05-31-2011, 10:39 PM   #13
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


A: Why do I feel like hitting Quiznos?

B: If you're gonna lose sleep over this, take a 1" bore bit and drill two holes per cavity through the double top plate into the attic. If you got a raised foundation, drill some holes through the bottom plate into the crawl space. Let convection dry your sandwich out.

Last edited by Calisota; 05-31-2011 at 10:41 PM.
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Old 05-31-2011, 11:39 PM   #14
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Quote:
Originally Posted by Calisota View Post
A: Why do I feel like hitting Quiznos?

B: If you're gonna lose sleep over this, take a 1" bore bit and drill two holes per cavity through the double top plate into the attic. If you got a raised foundation, drill some holes through the bottom plate into the crawl space. Let convection dry your sandwich out.
Wow.... cool. Just as I suggested (except for adding a length of PVC pipe to keep the insulation in the attic out of the way) and I'm not even a bath expert.
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Old 06-01-2011, 12:20 AM   #15
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


I would not suggest drilling holes in top or bottom plates. There's a fire stop issue there.

Basically, there should be no concern of a moisture sandwich. Use a membrane on the new bath and I don't see any reason there would be unwanted moisture getting in that cavity. It's an interior wall. You figure moisture or even vapor will have to penetrate the tile, grout, thinset, membrane, (more thinset if it's a fabric membrane) and CBU. It will be fine.
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Old 06-01-2011, 06:34 AM   #16
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Really, thanks everyone for getting into this.

Yes, I agree the surface treatments are much better than old poly.

I've always worked on the theory that almost no matter what you do, some small amount of moisture is gong to get back there, and plan for it. So this thread has had to alter my shower-wall paradigms a bit.

OK, in a simple residential bath/shower, we don't really expect vapour penetration to be a problem . We just have to seal out liquid.

Poly is good, but surface sealants are better.

Kerdi is pretty good, but NobleSeal is better.

I have poly, Hardi and Kerdi on-site. The thing's getting Kerdi'd.

Thanks again.
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Old 06-01-2011, 08:57 AM   #17
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Re: Choice: No Vapor Barrier In Bath, Or Mold Sandwich?


Every stud bay in your home should be able to breath from the outside.

You need to keep Vapour from getting through the warm side of the home and condensing in the cold stud bay.

Nobel blocks Vapour many times better than Kerdi. FACT.

Schluter does not care if you install poly behind your backer. They have advised a client of mine that this is fine.

To get really picky the tie in to a homes regular 6mil poly and Kerdi or Nobel TS is very hard.

Why not but poly over your insulation. Put Nobel, Kerdi, Hydro Ban what ever to 6' above grade. Any moisture getting into this cavity will in my opinion hydrate throw the hardi and breath out the top 1' of wall board and grout joints.

I do not think that any of these waterproofing products will all vapour transmission into a a wall assembly faster than regular evaporation and hydration can deal with them. In a commercial setting or steam room we would be debating Nobel over Kerdi. In this case you are not talking about a lot of water here.

JW

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